Genesis and metallogenic processes of the Potoumian Fe–Cu skarn deposit in western Guangdong: Evidence from garnet and magnetite geochemistry and Sr-Nd-Pb isotopes

• Garnet records fluid evolution from oxidized, acidic, open-system to reduced, neutral, closed-system. • Magnetite geochemistry indicates high-T magmatic-hydrothermal origin. • Sr-Nd-Pb isotopes of Magnetite link ore metals to a granitic magma source. • High W-Zn mineralization potential, while limited Sn prospectivity. The Potoumian Fe–Cu skarn deposit is located on the northwestern margin of the Yangchun Basin in the Yunkai area. The orebodies are hosted in calc-silicate hornfels, skarn, and skarnized limestone proximal to the contact zone between Middle Jurassic granodiorite and Upper Devonian carbonate rocks. Based on the detailed investigation, this study focuses on the elemental and isotopic characteristics of garnet and magnetite from the Potoumian deposit. Two distinct generations of garnet are identified: early-stage cores (Grt I) consist of andradite (And 92-96 Spe 3-4 Gro 1-5 ) showing LREE enrichment, HREE depletion, and strong to weak positive Eu anomalies; late-stage rims (Grt II) are grandite (And 59-64 Spe 5-10 Gro 30-32 ) exhibiting inverse REE fractionation (LREE-depleted relative to HREE) with pronounced negative Eu anomalies. Grt I is significantly enriched in Sc, Ti, V, Cr, Co, Ni, Ga, Ge, Rb, Sn, Nb, Ta, Zr, and Hf, but depleted in As, Sr, In, W, Cu, Pb, Zn, U, and REEs compared to Grt II. The geochemical evolution from Grt I to Grt II reflects a transition in fluid conditions: early mineralization fluids were characterized by an open system with high oxygen fugacity, high water/rock ratios, weakly acidic pH, infiltration-dominated metasomatism, and significant influx of external fluids (e.g., meteoric water). In contrast, later fluids evolved toward a more closed system with reducing conditions, lower water/rock ratios, diffusion-dominated metasomatism, and minimal external fluid input. Magnetite geochemistry suggests a hydrothermal origin, with primary mineralization occurring under high and stable oxygen fugacity at temperatures exceeding 300 °C, which is consistent with classification as a skarn-type deposit. Sr–Nd–Pb isotopic analyses of magnetite yield I Sr ratios ranging from 0.71089 to 0.71096, ε Nd (t) values from −8.2 to −8.3, and narrow ranges of 206 Pb/ 204 Pb (19.285–19.327), 207 Pb/ 204 Pb (15.851–15.874), and 208 Pb/ 204 Pb (39.526–39.623). These isotopic signatures indicate that the ore-forming fluids and metals were likely related to granitic magma. Additionally, this study highlights the significant potential for W and Zn mineralization within the deposit, while Sn mineralization appears limited. The findings provide key mineralogical and isotopic insights into the metallogenic processes of the Potoumian Fe–Cu polymetallic deposit.